The EIA-485 (also referred to as RS-485) standard defines a balanced multi-drop communication, which can be used by different types of serial communication protocols. EIA-485 is concerned with the electrical characteristics of the interface and does not specify a protocol, nor does it refer to a specific connector or collision detection or collision avoidance technique. A bus system according to EIA-485 can include up to 32 (unit load) devices and the length of the bus can be up to 1200 meters with Baud rates of 115200 bits/s or higher. Although EIA-485 is specified as Half Duplex (HD) transmission, the communication can become Full Duplex (FD) by applying a four wire connection.
Colliding messages can reduce the data throughput on communication links. For example, at least two messages can be lost in a collision. In addition, most protocols have re-transmission techniques in place, which can lead to further collisions. Accordingly, collisions should be avoided using suitable collision avoidance techniques such as Carrier Sense Multiple Access (CSMA) with Collision Avoidance. However, even with a collision avoidance measure, collisions can still occur and should be properly detected.
Within Substation Automation Systems, a protocol which can be used via, or over, EIA-485 is DNP 3.0, which can allow slaves transmitting messages in an uncoordinated manner (called “DNP unsolicited mode”). In this case, it is imperative to recognize collisions, as any data rate degradation on the bus is not acceptable, for example, if high priority event messages need to be sent.
FIG. 1 depicts a known HD setup with an EIA-485 transceiver connected to a bus including positive wire A and negative wire B. In order to relatively ensure a fail-safe EIA-485 communication, permanent pull-up and pull-down resistors RA, RB can be provided respectively between wire A and Vcc (e.g. 3.3 or 5 Volts) and between wire B and GND (ground), in order to bias the data lines into a defined state. A termination resistor RT can be used at the bus ends to help avoid reflections caused by high Baud rates and fast slew rates.
FIG. 2 depicts a known timing diagram for the transceiver wiring of FIG. 1, with ‘1’ being called ‘mark’ and ‘0’ being called ‘space’. As long as there is no traffic on the line (idle state), the line is transmitting marks or ‘1’ continuously. The start bit is ‘0’ (space) and the stop bit again is ‘1’ (mark). Therefore, there will be a transition from mark to space at the start of every word. This way the receiver can synchronise its clock regardless of the data content.
Many EIA-485 implementations can include a software-based collision detection principle where the transmit data TX_D, which was provided, for example, by a Universal Asynchronous Receiver/Transmitter (UART), can be compared against receive data RX_D recorded by the receiver during transmission of the transmit data TX_D. If the data does not correlate, an occurrence of a collision can be signalled. However, such a solution can be only suitable if the colliding signal superposes the transmit signal sufficiently. In an EIA-485 setup with a long transmission line exceeding a few hundred meters, and/or with a cable of bad quality, one transmitter can appear stronger than the other transmitter. Thus, a collision can remain undetected in case two transmitters are located at far ends of the bus.
FIG. 3 depicts an exemplary set-up of an EIA-485 bus system with a bus length of 1000 m and including two neighboring IEDs 1 and 2 as well as remote IED 32.
FIG. 4 shows the result of a transmission over the bus system of FIG. 3, wherein IED 1 is receiving, while IED 2 and IED 32 are both transmitting. The four signals displayed can include, from top to bottom, the transmitted signal from IED 2 (TX2_D), the signal transmitted from the remote IED 32 (TX32_D), a resulting superposition signal on the bus in the vicinity of IED 1 and IED 2 (Vbus), and the received signal at the RS-485 receiver inside IED 1 as obtained from Vbus by discrimination (RX1_D). As apparent from RX1_D, TX32_D is not strong enough to significantly influence Vbus. In other words, TX2_D can dominate and mask the collision with the messages from remote IED 32.
For example, it can be noted that the EIA-485 transceivers in IED 2 and IED 32 drive their signals with the same strength. The impedance of the transmission line, however, can reduce the signal energy from IED 32 as the signal travels from one bus end to the other. When arriving at IED 1 and IED 2, the signal may not be strong enough to superimpose the local signal.